SONOS (Silicon-oxide-nitride-oxide semiconductor) transistors are useful in memory circuits for providing non-volatile storage. However, SONOS transistors have traditionally operated in bias environments that tend to reduce the effective product lifetime and/or operating temperature of the SONOS transistors.
Biasing a SONOS transistor with an effective positive voltage on its gate will increase its ERASE threshold voltage. Likewise, biasing a SONOS transistor with an effective negative voltage on its gate will decrease its STORE (i.e. PROGRAM) threshold voltage. Both effects reduce the threshold voltage operating window of the SONOS transistor.
During non-volatile READ/RECALL, a stored non-volatile bit may be read to a bit line and sensed. During this process, the gate of a SONOS transistor may be biased in ways that tend to reduce the effective life or operating temperature of the transistor.
Due to capacitive effects, the gates of a SONOS transistor may remain in a biased state after a non-volatile operation (such as ERASE, RECALL, and STORE) when the memory circuit enters a “STANDBY” mode. The lingering bias voltage may over time cause drift in the voltage threshold window of the transistor.